1. Field of the Invention
This invention relates to compounds which can be used to increase sodium excretion in man or other mammals. The invention also relates to methods for purifying the compounds, and to pharmaceutical compositions and therapeutic methods utilizing the compounds.
2. History of Related Art
The adverse effects of high sodium concentration are considered to be at least partially responsible for a number of diseases, including hypertension, congestive heart failure, cirrhosis of the liver and renal disease leading to chronic renal failure. Understanding the nature of the system controlling sodium excretion, therefore, may lead to an effective therapy or cure for these diseases.
Over thirty years ago, de Wardener formulated that a "natriuretic hormone" must exist for sodium excretion regulation. In his classic parabiotic experiments in the dog, it was observed that prolonged increased sodium excretion (hereinafter referred to as "natriuresis") was produced in a normal animal by the infusion of plasma from a volume expanded animal. See de Wardener et al., Clin. Sci. 21, 249-258 (1961). Since then, considerable effort has been employed by researchers in the field to isolate and identify the factors involved in the regulation of sodium excretion.
Most investigators in this field believe that this putative humoral substance may be responsible not only for sustained natriuresis, but also for inhibition of sodium transport and increased vascular reactivity. The latter two effects occur via inhibition of the Na.sup.+ /K.sup.+ -ATPase, also known as the sodium pump. Therefore, many in vitro assays have been used to search for the hormone based on the tenet of sodium pump inhibition by the hormone. These assays include Na.sup.+ /K.sup.+ -ATPase inhibition, ouabain displacement from the pump, and cross-reaction of isolates from various in vivo sources with anti-cardiac glycoside antibodies. (See Wechter, et al., Prog. Drug Res. 34, 231-260 (1990). Research has focused on isolation of natriuretic factors acting on the Na.sup.+ /K.sup.+ -ATPase pump. Numerous putative natriuretic compounds (sometimes identified as hormones) have been found using the above-mentioned methodologies. See, e.g., Wechter et al., Prog. Drug Res. 34 231-260 (1990). For example, Bricker et al., U.S. Pat. No. 5,106,630 identified a compound having a steroidal nucleus, a molecular weight of 360 and a molecular formula of C.sub.21 H.sub.28 O.sub.5 as natriuretic hormone.
Ouabain and digoxin or their isomers having been isolated recently using these methodologies. These materials are not, however, natriuretic. See Ludens et al., Hypertens. 17, 923-929 (1991); Mathews et al., Hypertens. 17, 930-935 (1991); Tymiak et al., Proc. Natl. Acad. Sci. (USA) 90, 8189, 8193 (1993); Goto et al., Biochem. Biophys. Res. Comm. 173, 1093-1101 (1990). These and other results indicate that sodium pump inhibitory activity does not necessarily lead to natriuresis. In fact, it is at least as likely to produce kaliuresis. See Sekihara et al., Life Sci. 53, 975-980 (1993); Pamnani, et al., Hypertens. 18, 316-324 (1991); Smyth et al., Can. J. Physiol. Pharmacol. 70, 723-727 (1992); Crabos et al., Eur. J. Biochem. 162, 129-135 (1987). Therefore, a natriuretic compound purification process solely using assays based on binding to and/or inhibition of the Na.sup.+ /K.sup.+ -ATPase pump may not lead to the discovery of the putative natriuretic hormone.
In addition, despite decades of efforts, natriuretic compounds had not been isolated in pure form, chemically and structurally defined, or synthesized in the laboratory. Also, as demonstrated above, the above-mentioned methodologies used in the purification process to monitor natriuretic activity fail to measure natriuresis directly. Therefore, prior to the present invention, the putative natriuretic hormone had not been identified.
It would be highly desirable to provide compounds having a natriuretic effect. It would also be desirable to develop a new method to isolate endogenous natriuretic compounds by using an assay measuring natriuresis directly. It would be desirable as well to develop a more effective purification method to obtain substantially pure natriuretic compounds in an amount sufficient for chemical and structural definition.